1. Field of Endeavor
The invention relates to a premix burner for producing an ignitable fuel/air mixture, including a swirl generator which provides at least two burner shells which complement one another to form a throughflow body, which in each case have a first burner shell section designed in a partial cone shape and together enclose an axially conically widening swirl space and which mutually define in the axial cone longitudinal extension tangential air inlet slots, through which the combustion feed air passes into the swirl space, in which an axially spreading swirl flow forms, and comprising a device for spraying fuel arranged at least in sections along the tangentially running air inlet slots.
2. Brief Description of the Related Art
Premix burners of the abovementioned generic type are known from a large number of publications, for example, from EP 0 210 462 A1 and EP 0 321 809 B1, to mention only a few. Premix burners of this type are based on the general operating principle of generating a swirl flow consisting of an air/fuel mixture inside a usually conically designed swirl generator which provides at least two partial cone shells assembled with a correspondingly mutual overlap, and this swirl flow is ignited inside a combustion chamber following the premix burner in the direction of flow, with a premix flame being formed which is spatially as stable as possible. In this case, the spatial position of the premix flame is determined by the aerodynamic behavior of the swirl flow, the swirl coefficient of which increases with increasing spread along the burner axis and thus becomes unstable and ultimately breaks down into an annular swirl flow due to a discontinuous transition between burner and combustion chamber, with a backflow zone being formed, in whose front region in the direction of flow a premix flame forms.
Of particular importance is the aerodynamic stability of the forming backflow zone, which, however, depends in a most sensitive manner on the design, shape and size of the swirl generator. For example, if it is not possible to spatially stabilize that part of the forming backflow zone which is right at the front in the direction of flow, thermoacoustic vibrations or pulsations occur to an intensified degree within the combustion system and considerably impair the entire combustion and the emission of heat.
In view of these facts, the hitherto known premix burners in use are restricted to sizes whose maximum burner diameter at the burner outlet is only 180 mm. In addition, such premix burners have a relatively acute, i.e., small, cone angle less than or equal to 18°, so that the burner length in relation to the downstream burner diameter is rather on the large side but can still be readily manipulated for fitting or maintenance purposes.
However, if it is necessary to fire combustion chambers of large dimensions, “multiple burner arrangements” which provide for the use of the above premix burners have been used hitherto. Such multiple burner arrangements of complex construction have been disclosed, for example, by DE 42 23 828 A1 or DE 44 12 315 A1. However, it is desired to reduce the complexity and thus also the number of the individual premix burners, required for firing combustion chambers of large dimensions, without at the same time having to tolerate quality losses in the combustion process itself. In addition, for reasons of environmental standards, which are always becoming stricter, with regard to the reduction of emission figures, it is necessary for the individual diffusion burners used hitherto, which are mainly used for firing silo-type combustion chambers of large dimensions, to be replaced by modern burner systems which are more environmentally compatible. In particular with regard to the avoidance of high conversion and new-procurement costs, it is desirable to provide premix burners of the largest possible dimensions in order to be able to continue to maintain, for example, the operation of such silo-type combustion chambers of large dimensions with only a single premix burner.
Theoretical considerations and tests have shown that simple scaling, for example, of a double cone burner known from EP 0 321 809 B1, is not successful, especially since, as already mentioned above, the burner length would increase disproportionately. There is also the fact that the width of the air inlet slots which run tangentially in the burner axis and through which the combustion feed air for generating the desired swirl flow flows into the swirl generator would likewise increase proportionally, so that good intermixing of fuel and combustion air can no longer be ensured to a sufficient quality.
In most premix burners in use, the partial cone shells which are provided for deflecting and guiding the feed air into the swirl generator and which may also be referred to as burner shells are designed as thin-walled baffle plates which have the shape of the lateral surface of cone halves or smaller cone segments and radially define the swirl space, the burner shells, due to their spatial arrangement, in each case jointly enclosing air inlet slots mutually oriented tangentially to the burner axis.
In endeavors to improve the absorption and output capacity of such premix burners, swirl generators having more than two burner shells are known, “multi-shell premix burners”, which can also ensure a larger burner diameter. However, it has been found that no satisfactory intermixing results are obtained with such multi-shell arrangements, especially since aerodynamic problems occur which in all probability can be attributed to backflow zones forming locally in the region of the individual burner shells. This leads firstly to efficiency losses, but also entails risks if combustible fuel can collect in such backflow zones and ultimately ignite.
U.S. Pat. No. 6,702,574 B1 discloses a burner for operating a heat generator, including a swirl generator whose inlet cross section oriented in the direction of flow is of rectangular design and provides downstream, for reasons of improved intermixing, a throughflow cross section which is square or round in the direction of flow and preferably adjoining which is a mixing section of round cross section. Shown in a perspective view in the exemplary embodiment according to FIG. 3 thereof is a swirl generator whose swirl space is radially defined by burner shells 156 of curved design. The burner shell or swirl blade 156 shown in cross section has three burner shell sections which are connected to one another in one piece, a second burner shell section curved in opposition to the first burner shell section, in each case designed in a partial cone shape being added in a flush manner to the first burner shell section, and a third burner shell section adjoining the second burner shell section in a flush manner, the third burner shell section having a curvature tangentially adapted to the second burner shell section.